1. Field Of The Invention
This invention pertains to the field of separating particular constituents dissolved in a liquid medium by the use of semi-permeable membranes. More specifically, the present invention relates to the separation of ethylene glycols from at least one aliphatic organic acid having one or more carbon atoms or an alkali metal or alkaline earth metal salt thereof, at least one ultraviolet-light absorber or its precursor, or combinations thereof. The invention is particularly useful in the process for making polyester-grade monoethylene glycol from ethylene oxide.
2. Discussion Of Related Art
The preparation of ethylene glycol is of particular interest to the chemical industry because of the varied uses of this compound. A particularly important use of ethylene glycol is in the production of polyester fibers. Ethylene glycol used in the manufacture of polyester fibers generally must be of exceptionally high purity because even a small quantity of impurity may have a deleterious effect on the resulting polyester fiber. Ethylene glycol also finds application in heat-transfer fluids, deicing fluids, antifreeze, hydraulic fluids, and in the manufacture of alkyd resins and solvents.
The ability to increase the purity of ethylene glycol product is of particular interest for the manufacture of polyester fiber. When ethylene glycol contains even small quantities of impurities, the properties of the polyester produced, such as fiber dyeing characteristics, fiber strength, fiber color, etc., generally are affected adversely. High purity ethylene glycol suitable for use in the product of polyester fiber is referred to as polyester-grade ethylene glycol.
In order for ethylene glycol to qualify as polyester-grade ethylene glycol, it must pass a stringent ultraviolet light (UV) transmittance test. This test is conducted by comparing the transmittance of ultraviolet light at designated wavelengths through samples of ethylene glycol and of distilled water. The amount of ultraviolet light transmitted through a sample of distilled water of similar thickness, converted to a percentage, constitutes the percent transmittance of an ethylene glycol sample. Current commercial fiber-grade ethylene glycol ultraviolet light transmittance specifications are set forth below:
______________________________________ Ultraviolet Light Wavelength Minimum Percent (nanometers) Transmittance (%) ______________________________________ 220 70 275 90 350 98 ______________________________________
As used herein, the term "UV absorbers" refers to materials which, when present in a sample of ethylene glycol, undesirably reduce the transmittance of ultraviolet light through the sample. The term is also meant to include materials which are UV absorber precursors, that is, materials which by themselves do not reduce the transmittance of ultraviolet light but which, when present during the preparation of the monoethylene glycol, are transformed to such UV absorbers. The reduction in the UV transmittance of an ethylene glycol sample is therefore a measure of the purity of that sample. In other words, the greater the UV transmittance of an ethylene glycol sample, the purer it is and the more valuable it becomes. Accordingly, it is preferable that the ethylene glycol not only satisfy the UV transmittance standards identified above, but that its UV transmittance be as high as possible.
Ethylene glycols (monoethylene glycol, diethylene glycol, triethylene glycol and tetraethylene glycol) are prepared commercially by several methods well known to those skilled in the art. One of these methods involves a two-stage reaction system, the first stage of which requires the direct oxidation of ethylene with air or elemental oxygen over a suitable catalyst, typically a silver-containing catalyst, at elevated temperature (100.degree. C. to 500.degree. C. is typical) and at superatomospheric pressure (2 to 25 atmospheres).
Ethylene oxide produced in these reactors, which may be fixed or fluid bed reactors, as typified by U.S. Pat. Nos. 2,125,333, 2,430,443, 3,904,656 and 3,970,711, is removed from the reactors in a gas stream and is passed into an ethylene oxide scrubber where the gas stream is contacted with water to absorb the ethylene oxide content thereof. The gases leaving the scrubber as overhead (which still contain appreciable quantities of ethylene) are then recycled to the ethylene oxide reactor. The scrubber bottoms, i.e., the ethylene oxide containing water is then passed to a stripper. In the stripper, steam or hot water may be introduced and contacted, usually countercurrently, with the ethylene oxide fed to the column to remove ethylene oxide product overhead. Alternatively, the ethylene oxide containing water may be subjected to temperature and pressure conditions within the stripper which remove the ethylene oxide as overhead. The water discharged from the stripper as bottoms is generally recirculated to the scrubber for reuse in absorbing ethylene oxide.
Inasmuch as water is constantly being generated in the ethylene oxide reactor at the same time as the aqueous stripper bottoms are being recycled to the scrubber in a closed system, a purge stream is required to remove the excess water which accumulates. This purge stream, however, generally contains appreciable quantities of ethylene glycols, typically up to about 10 percent by weight. Most of the contained glycols consist of monoethylene glycol (about 0.1 to about 10.0% by weight) with the remainder being diethlylene glycol (about 0.01 to about 1.0% by weight), triethylene glycol (about 0.001 to about 0.1% by weight), and trace levels of higher molecular weight glycols. As used herein, the term "ethylene glycols" is meant to include mono-, di-, and triethylene glycols as well as higher molecular weight glycols. The amount of glycols in this purge stream is of such value that it is not economically preferable to simply discard it. Hence, it may be introduced to a glycol reactor in which ethylene oxide and water are reacted to form ethylene glycols or into the refining train downstream of the glycol reactor. The ethylene glycols produced in the glycol reactor are first passed to an evaporator wherein water is removed. The glycols product is taken from the evaporator as bottoms, and then passed through a distillation train to produce refined monoethylene glycol, the desired product, as well as diethylene glycol, triethylene glycol, tetraethylene glycol, etc. as by-products.
Frequently, however, the purge stream which contains the valuable glycols and which is introduced into the glycol process, may contain various impurities such as aliphatic organic acids having one or more carbon atoms and/or the alkali metal or alkaline earth metal salts thereof. Some of these acids and/or salts may comprise UV absorbers or their precursors. The purge stream may also contain one or more high molecular weight derivatives of ethylene oxide other than glycols which may also be UV absorbers or precurors which form UV absorbers during subsequent processing steps. Consequently, these impurities are deleterious to the monoethylene glycol product that is intended to be produced, preventing it from being classifiable as polyester-grade.
A need accordingly exists for providing a technique in which undesirable UV absorbers and/or their precursors which are contained in glycol-containing purge streams, such as the purge stream from the aqueous bottoms of the stripper, are economically and efficiently separated from the valuable glycols.